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InterviewSolution
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Why Is A Union All Faster Than A Union? |
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Answer» The UNION operation, you will recall, BRINGS two sets of DATA together. It will *NOT* HOWEVER produce duplicate or redundant rows. To perform this feat of magic, a SORT operation is done on both tables. This is obviously computationally intensive, and uses significant memory as well. A UNION ALL conversely just dumps collection of both sets together in random ORDER, not worrying about duplicates. The union operation, you will recall, brings two sets of data together. It will *NOT* however produce duplicate or redundant rows. To perform this feat of magic, a SORT operation is done on both tables. This is obviously computationally intensive, and uses significant memory as well. A UNION ALL conversely just dumps collection of both sets together in random order, not worrying about duplicates. |
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